The problem of synchronization of network nodes or base station entities in a mobile communication network relates to the question of how radio access nodes (or base station entities) can be efficiently synchronized against each other in order to use radio resources in a more efficient way. In principle, this applies to all radio access technologies. Some modes of the LTE technology require an especially tight network synchronization in order to operate efficiently: The TDD (Time Division Multiplex) mode of operation is such an example where the timing, or at least the timing difference between two nodes (or base station entities) needs to be ensured (in an accurate manner) in order to allow the user equipment to use a defined timing for its operation.
Multiple methods of adjusting time drift between different radio nodes (or base station entities) are known today.
These methods include GNSS (Global Navigation Satellite System) based methods where the synchronization is performed to an external reference, normally provided by the GNSS system clock (such as a GPS system clock). These methods require the base station entities to be equipped, respectively, with suitable receivers (e.g. GPS receivers), which are, on the one hand, costly to purchase and install and, on the other hand, only (or mainly) usable outdoors.
As the network nodes, not only of a mobile communication network but of computer networks in general, need to be tightly synchronized in order to be able to properly work, a number of methods to realize the synchronization of network nodes have been proposed: E.g., the Network Time Protocol or the Precision Time Protocol.
The Network Time Protocol (NTP) is a networking protocol for clock synchronization between network nodes of a computer systems over packet-switched, variable-latency data networks and is intended to synchronize all participating computers to within a few milliseconds of a given time base. For synchronization purposes between the network nodes of a mobile communication network, this accuracy of a few milliseconds is not sufficient.
The Precision Time Protocol (PTP)—also known as IEEE 1588 is a protocol used to synchronize clocks of connected network nodes of a computer system such as the base station entities of a mobile communication network. Using the Precision Time Protocol, it is possible to attain accuracies beyond those attainable using NTP or for applications without access to a GPS (Global Positioning System) signal.
However, using the Precision Time Protocol requires using within each network node of the mobile communication network that needs the precise time base, i.e., within each base station entity—a precision clock, hence a comparably expensive hardware component (being able to maintain a certain timing reference with a certain accuracy).
When using the PTP for mobile communication networks to adjust time drifts between base station entities, a reference timing from a server is used to convey this timing information via the backhaul to the respective radio nodes. However, the advent of small cells—also for usage indoors—places new requirements on the synchronization methods as GPS does normally not work indoors or is simply too costly and the PTP (or IEEE1588) might not always be available over the low tier backhaul connections some (indoor) small cells are connected to.